A heater core is a heat exchanger that transfers heat from engine coolant to flowing air in a heating ventilation and air conditioning (HVAC) unit of an automobile. Liquid engine coolant is pumped through coolant paths in an internal combustion engine to carry waste heat from the engine and keep the engine within operational temperature limits. A heater core may be installed in the coolant path and in an airflow path within the HVAC unit. A fan may blow air through the heater core that has been warmed by the engine coolant. As the air passes through the heater core, the engine waste heat is transferred from the liquid engine coolant to the air, thereby raising the temperature of the air. The heated air is ducted to the passenger compartment of the vehicle to raise the temperature of the air in the passenger compartment.